Rapid Fabrication of High-Performance Flexible Pressure Sensors Using Laser Pyrolysis Direct Writing

The fabrication of flexible pressure sensors with low cost, high scalability, and easy fabrication is an essential driving force in developing flexible electronics, especially for high-performance sensors that require precise surface microstructures. However, optimizing complex fabrication processes and expensive microfabrication methods remains a significant challenge. In this study, we introduce a laser pyrolysis direct writing technology that enables rapid and efficient fabrication of high-performance flexible pressure sensors with a micro-truncated pyramid array. The pressure sensor demonstrates exceptional sensitivities, with the values of 3132.0, 322.5, and 27.8 kPa–1 in the pressure ranges of 0–0.5, 0.5–3.5, and 3.5–10 kPa, respectively. Furthermore, the sensor exhibits rapid response times (loading: 22 ms, unloading: 18 ms) and exceptional reliability, enduring over 3000 pressure loading and unloading cycles. Moreover, the pressure sensor can be easily integrated into a sensor array for spatial pressure distribution detection. The laser pyrolysis direct writing technology introduced in this study presents a highly efficient and promising approach to designing and fabricating high-performance flexible pressure sensors utilizing micro-structured polymer substrates.


Fabrication of the PDMS Film
The liquid mixture of PDMS precursors (Sylgard 184, Dow Corning, mixture ratio: 10:1) was poured onto a smooth plastic template and placed inside a vacuum chamber for degassing.
The mixture was then cured at 80 °C for 2 hours to form a 2 mm thick transparent PDMS film.

Fabrication of the Microstructure Arrays
The micro-truncated pyramid arrays were fabricated using laser pyrolysis direct writing technology on the PDMS surface.This was achieved by using an ultraviolet (UV) pulsed laser system (Grace X 355-3A, Han's Laser Technology Industry Group Co., Ltd., wavelength: 355 nm) operating at a fixed scanning speed ( ) of 10 mm/s, a laser average power ( ) of 1.5 W, and a pulse repetition frequency ( ) of 40 kHz.SiC nanoparticles (Bide Pharmatech Co., Ltd., particle size: 0.5-0.7 μm) were coated at the initial point of the scan path to induce the continuous laser pyrolysis reaction.After fabrication, the pyrolysis products (3C-SiC) on the PDMS surface were cleaned with an ultrasonic cleaner using anhydrous ethanol and dried with nitrogen gas.A 150 nm thick gold conductive layer was deposited onto the PDMS film (12 × 12 mm) with micro-truncated pyramid arrays using magnetron sputtering equipment (KT-Z1650PVD, Zhengzhou Ketan Instrument Equipment Co., Ltd.)

Packaging of the Flexible Pressure Sensors
The commercial single-sided conductive tapes (3M7766-50, 3M Company) were utilized as lead wires to connect the ITO/PET electrode (South China Xiangcheng Technology Co., Ltd, thickness: 125 μm, resistance: ≤10 Ω/sq) and Au/PDMS electrode, respectively.Liquid metal (Alfa Aesar Chemical Co., Ltd., metal basis: 99.99 % Ga-In-Sn) was coated on the conductive tapes.Subsequently, the ITO/PET electrode and Au/PDMS electrode were aligned and assembled by heating a heat-shrinkable tube at 100 °C to complete the final packaging of the flexible pressure sensor.

Characterization and Measurement
The morphology of the PDMS film with micro-truncated pyramid arrays was characterized using a scanning electron microscope (SEM, Gemini 300, ZEISS) and a 3D laser scanning microscope (VK-X1000, KEYENCE).Thermal gravimetric analysis (TGA, STA 449C, NETZSCH) was employed to investigate the changes in gravimetric components during the pyrolysis of PDMS.The samples were heated from 27 °C to 1400 °C at a ramp rate of 20 °C/min in air.The pyrolysis products were characterized using X-ray diffraction (XRD, Rigaku Smartlab) with a scanning rate of 10°/min.The mechanical performance of the flexible pressure sensor was evaluated using a universal testing machine (TSE503A, Wance Testing Machine Co., Ltd.).The corresponding electrical signals were measured using a digital source meter (2450, Keithley), and the response time was recorded using an oscilloscope (TBS2104B, Tektronix).

Supplementary Discussion 2.1
In detail, the laser beam acts as an electromagnetic wave that carries both energy and momentum.When it interacts with the PDMS surface and 6H-SiC NPs, it not only provides energy to the 6H-SiC NPs at the initial position through the photothermal effect but also generates radiation pressure on the PDMS surface.The high photon density of the laser causes the optical pressure at the focal plane to be significantly higher than the natural optical pressure of 0.5 dyne 1 .Furthermore, radiation pressure causes the state of 6H-SiC on the PDMS surface to be unstable, making it difficult to accumulate heat and trigger continuous laser pyrolysis

Supplementary Discussion 2.2
In detail, increasing the average power ( ) of the laser at the fixed repetition frequency ( ) and scanning speed ( ) did not effectively improve the aspect ratio of the microchannels, which remained stable at around 1.55, as shown in Table S1.On the other hand, the width and depth of the microchannels increased simultaneously with the laser power.It is worth noting that the final morphology of the microchannel is jointly determined by the critical pyrolysis temperature of continuous laser pyrolysis and the morphology of SiC pyrolysis products.The overall temperature during laser pyrolysis significantly increases as the laser power increases.
However, heat conduction in the medium remains isotropic 2 .The temperature distribution in the system formed by the PDMS substrate and SiC pyrolysis products still follows the Gaussian temperature distribution.As the laser power increases, the temperature of the laser-irradiated SiC pyrolysis product also rises.Due to the constant thermal conductivity of the SiC pyrolysis product, its overall temperature increases and maintains isotropic conduction.However, the critical temperature for laser pyrolysis remains unchanged.Therefore, while the laser moves, the cross-section of the PDMS microchannel expands uniformly in all directions.Consequently, increasing the laser power leads to an increase in the depth and width of the SiC pyrolysis product morphology, which also keeps the aspect ratio of the microchannel unchanged.In contrast, increasing the number of laser scans gradually increases the etching depth of laser scanning due to the high temperature and stress generated during the pyrolysis process, as shown in Figure S5.As the etching depth increases, the depth of the PDMS microchannels after laser pyrolysis increases, which leads to an increase in the aspect ratio of the microchannels.

Supplementary Discussion 2.3
In detail, Figure S6 and Figure S7 show the roughness results and 3D laser confocal images of the microchannel sidewalls with different numbers of laser scans.To avoid measurement errors resulting from the groove-like shape of the microchannel cross-section, line roughness analysis was conducted on the roughest position of the microchannel sidewall for varying numbers of laser scans.The average roughness (Ra) of the microchannel sidewall increased from 0.2 μm to 0.38 μm and 0.60 μm with an increase in the number of scanning times.
Meanwhile, the average roughness depth (Rz) also increased from 0.92 μm to 1.94 μm and 3.78 μm, respectively.These results clearly show that the microchannel roughness increases with multiple laser scans compared to a single laser scan.This is because as the number of laser scans increases, the 3C-SiC pyrolysis product extends further into the PDMS, leading to an increase in thermal expansion and interfacial stress between the PDMS substrate and the 3C-SiC pyrolysis product.As a result, the interlayer dislocation between the 3C-SiC pyrolysis products and PDMS results in an increase in the surface roughness of the PDMS microchannels.

Mechanism Analysis of LPDW Technology
Figure S8.The pyrolysis mechanisms of PDMS under low-heating-rate pyrolysis and high-heating-rate pyrolysis routes.

Supplementary Discussion 2.4
The schematic diagram of the pyrolysis mechanism under high and low heating rate conditions is illustrated in Figure S8.In fact, the conversion mechanism of PDMS to 3C-SiC through continuous laser pyrolysis is influenced by various physical conditions.Burn et al.
reported on the conversion of siloxane polymers to silicon carbide, which provided a general theory for the pyrolysis of polysiloxane-based materials through a two-step successive pyrolysis 3 .Although their research mainly focuses on siloxane polymers rather than PDMS, it is still informative.Subsequently, Camino et al. conducted further investigations on the pyrolysis of PDMS through experiments and simulations, which revealed two competing mechanisms of molecular (Low heating rate pyrolysis route) and radical mechanisms (High heating rate pyrolysis route) in the pyrolysis behavior of PDMS at different heating rates 4,5 .
Based on the above, Shin et al. proposed a reaction path for laser pyrolysis of PDMS that directly converted it into SiC at low temperature 6 .
However, the comparative studies on laser pyrolysis products and processes still need to be clarified.Therefore, we compared and summarized the low-heating-rate and high-heating-rate pyrolysis routes.When the heating rate is lower than 50 °C/min, PDMS is pyrolyzed into various cyclic oligomers at around 500 °C.In this phase, the molecular mechanism dominates and causes the breaking and reforming of Si-O bonds on PDMS chains to form cyclic oligomers.
This further induces tight cross-linking of PDMS chains, leading to an increase in thermal stability.Subsequently, under conditions of low chain flexibility, the cyclic oligomers decompose into SiC with a further increase in temperature.When the heating rate is higher than 100 °C/min, PDMS is pyrolyzed into more stable carbide or oxycarbide of silicon in the low-temperature range of 600 °C to 700 °C.The radical mechanism dominates and inhibits the molecular mechanism in this phase, inducing the breaking and reforming of Si-CH3 bonds on PDMS chains to increase the additional cross-linking of PDMS further.This mechanism also leads to the condensation of the remaining main chains, resulting in the derivation of silicon oxides.Finally, the oxides of silicon are reduced by the pyrocarbon in the absence of oxygen to produce silicon carbide (SiC) and carbon monoxide (CO).As the incident laser energy is absorbed and partially converted into heat, transient heat equation for the spatial distribution( , , )of temperature ( , , , ) at time as definedifference form:

Thermal and Mechanical Effects LPDW Technology
the corresponding initial condition is written as: ( , , , = 0) = = 293.15K (S7) the boundary conditions are formulated as: where the , , and are density, heat capacity, and thermal conductivity of the material in the heat conduction system, respectively.and are the external temperature and ambient temperature, respectively, in the equation (S7) and (S8).Additionally, ℎ, , and are the heat transfer coefficient, emissivity, and Boltzmann constant 12,13 .The heat transfer coefficients for the top and bottom surfaces of the material are determined using empirical formulas, resulting in values of 17.9 W/m 2 /K and 9.0 W/m 2 /K, respectively 14 .These coefficients are used to characterize the heat exchange process between the material and the surrounding air during natural convection process.

Supplementary Discussion 2.6
In addition, much attention has been paid to investigating the deformation and stress resulting from the photothermal effect during continuous laser pyrolysis.Therefore, we conducted a thermomechanical coupling analysis of continuous laser pyrolysis.To simulate changes in stress and deformation using the finite element method (FEM), certain assumptions were applied in the computational model, as follows: (1) The mechanical properties, stress, and strain of the material changed linearly during a smalltime increment.
(2) In the plastic zone, the material followed the hardening rule and flow rule.
(3) The material underwent plastic deformation following the law of constant volume.
(4) The yield deformation process of the material followed the Von Mises yield criterion.

Figure S1 .
Figure S1.Schematic illustration of continuous laser pyrolysis surface reaction morphology at different pulse repletion frequencies (low and high frequency).

Figure S2 .
Figure S2.3D laser confocal images of microchannel structures on PDMS surface at different focal distances.

Figure S3 .
Figure S3.Schematic illustration of the relationship between the laser average power ( ), laser peak power () and pulse repetition frequency ( ).

Figure S4 .
Figure S4.The detailed heat map of critical realization conditions for continuous laser pyrolysis (CLP) reaction.

Figure S5 .
Figure S5.The corresponding scanning electron microscopy (SEM) images of the surface topography under the different numbers of laser scans (N1, N2, and N3).

Figure S9 .
Figure S9.Schematic illustration of laser pyrolysis parameters in the defocused condition.

Figure S10 .
Figure S10.The functional relationship between the spot radius ( ( )) and the defocus distance ( ) in the defocused state.

Figure S11 .
Figure S11.(a) 3D isothermal surface distribution of the initial pyrolysis process and corresponding top view (b) and cross-sectional views (c) and (d) (Y = 0 μm and X = 500 μm).

Figure S12 .
Figure S12.3D and cross-sectional distributions of the deformation evolution of PDMS and SiC during continuous laser pyrolysis (Time = 0.025, 0.050, and 0.075 s).

Figure S13 .
Figure S13.3D and cross-sectional distributions of the stress evolution of PDMS and SiC during continuous laser pyrolysis (Time = 0.025, 0.050, and 0.075 s).

Figure
FigureS13shows the 3D and cross-sectional distributions of the deformation evolution

Figure S14 .
Figure S14.The principal stress line distributions of PDMS and SiC during continuous laser pyrolysis (Time = 0.025, 0.050, and 0.075 s).

Table S1 .
The average width and depth of the microchannel with different average power at fixed pulse repetition frequency.